BuckyE wrote:
It seems to me a set of concentric ridges would not mill very well.
I'm no expert either, but I can still see those ridges as indcatig millstone-ness. There's nothing to indicate that the centre of the stone was in what would be the top left of SL's photo.If the hole of this putative millstone were at the top right, then the grooves would be placed in such as way as to do the business with the grain.